Vibrato means for audio amplifiers



Dec. 19, 1950 N. l/DANIEL VIBRATO MEANS FOR AUDIOAMPLIFIERS 2 Sheets-Sheet 1 Filed Aug. 29, 1947 FDA-Z- JJDL Ina ATTORNE Dec. 19, 1950 Filed Aug. 29, 1947 2 Shets-Sheet 2 4 gm 1 m0 I, 3 S S 3 1,. an .n n W ma k A @n I n" Pan! 32-1331 v #T m ATTDRNEY.

' Patented Dec. 19, 1950 STAT VIBRATO MEANS FOR AUDIO" AMPLIFIERS Nathan 1. Daniel, Long, Branch, N. J.

7 Application August 29; 1947, Seria't-No'. 771,259

4 Claims. 1

Thisinventionrelates: to audio frequency amplifierdevices and more particularly to an amplifier for a musical instrument or the. like wherein the output is automatically. provided with a tremolo or vibrato effect.

It iswell recognized thata vibrato improves the quality of'music and it is employed by accomplished musicians through supplementary manipulationof the strings or other soundproducing elements-of aninstrument. However, not only must the musician be. accomplished at his task, but it requires, particular efiort and attentionwhile the human element involved often causes-notes or: passages to .be deprived of vibrato or to be insufficiently provided therewith.

Bearing-lthe foregoin in mind, it. is an object offtheapresent'invention to provide an amplifier for musical instruments .or any other sound producingmedium wherein the sound output is automatically provided with a, Vibrato effect and in which thefrequency. of the vibrato can be simply and effectively controlled; and further where the vibrato frequencyisz not heard as. separate notes in the amplifier output. It. will be. understood that a vibrato effect is a more or less continuous and'regular'modulationoi the amplitude of .the music. If; for example, the-rate of modulation wasofthe. usual frequency of aboutB'to 20 times per-xsecond, ,careshouldbe. taken. that anote of this frequencyais: notemitted as otherwise a. quite foreignsound would be evident, generally by a thumping noise. The instant apparatus is effective therefore, in producing the required amplitude variations to add vibrato while suppressing the vibrato frequency per se from the amplifier outputandaccomplishing the same electrically orelectronically and without the use of any moving parts;

The; invention further; comprehends, in a slightly. modified embodiment, means for producingaqhighdegree of vibrato without introducing;distortion in the amplifier output. By a high degree of vibrato is meant that the sound intensity; can be caused to vary from very low to veryhigh limits during the rendition of a. note or ;passag.e-- V r Otherobjects'of my invention will be apparent from thefollowing description, it being understood that the above general statements of the objects of myinvention are intended to describe and not limitit in any manner.

Referring, to the drawings:

Fig. lis a schematic view of vibrato: means incorporated into an audio amplifier according. to the-presentinvention.

Fig; 2 is a' modifled embodiment thereof;

Theamplifier disclosed-is provided with aflconventional push-pull output comprising a pair: of outputtubes [ii and 11.. According to the usual arrangements of push-pull amplifiers, the outiputs'of tubes' I tl and i I are .combinedin thetrans-1- former l2 inad-diti've. fashion while therinputsato the control grids I 3 and I tare fed. by oppositely.

phased signals froma prior push-pull or'phaseinverter-stage as'is' conventional. Tubes l0: and Ms are illustrated as: including." screen. grids. I51 and 1.5a such as are. found. in. tube-'- type; 6L6; although any suitable: outputtube'; such: as. a

triode (type 6A3): or. axpentode (.typeri'l) may;

well be employed. Thespeaker". IG- reproduces and. emitszthev sound-waves :through" the currents produced in the output circuit. Resistors l1 and- [8c are the; usual grid .leaks,.bias being. secured through a O-bias filter: l9 in: the power supply, of theamplifier, although any conventional bias systemmay; be employed. Resistor 2n isan isolating resistor of approximately 100,000 ohms;

Thev power supply comprises the usual input transformer 2| and rectifier tube 22 incorporated.

in-.a: conventional rectified 'power supply andfilter: arrangement. In the instant embodiment,

(the speaker fieldv winding 23 is connected across the power supply so as to: function .as a blower whilea tap therefrom supplies. voltage-tothe screen grids through wire 24.

The. foregoing represents-a moreor less con-. ventional type of amplifier provided :with apushpull output. With the abovecircuit, I have com+ binedaavibrato;meansso that: the sound fre quency output of speaker [G is provided with the desiredpvibrato efiect. This is accomplished by; the use of-an audio oscillator the output of which. isrconnected-lto the control gridsl3 and, M of the output tubes. variety of forms which can be found in any radio handbook. Any, audio oscillator can perform the tion .ofsfrequencyi through :the potentiometer 29; theconstan b g; chosen: to produce an out? Audio oscillators may take a put of approximately 3 to cycles per second. Accordingly, resistors 21 and 28 may be 50,000 and 600,000 respectively while potentiometer 29 is 1 megohm. Condensers 30, 3| and 32 may be .02 mfd. Resistors 26 and 33 may be 390,000 ohms each, and grid leak and phasing resistor 34 may be 1.2 megohms. Screen by-pass condenser 35 may be in the order of .5 mfd. The output of the tube is taken across 500,000 ohm potentiometer 36 through the isolating condenser 31 and resistor 38. It will be understood that all the foregoing values are merely illustrative and may be varied within wide limits in producing the desired frequency range. Wire 39 applies the vibrato oscillator output through condenser 40 to the circuit midpoint 41 of the grids l3 and I4. Consequently, the vibrato output appears as a regular fluctuating bias on the grids of the output tubes. The amplitude of such fluctuating bias signal is secured through manipulation of potentiometer 35 while the frequency is controlled as aforesaid through potentiometer 29.

It will be evident that whereas the normal audio signals from the input of the amplifier are fed to the grids l3 and [4 in an out-of-phase arrangement, the vibrato voltages are fed to the grids in parallel or in the same phase. This sets up a. condition for balanced modulation where the frequency. of the vibrato oscillator may be suppressed or cancelled in the output circuit appearing only as an amplitude modulation of the output voltages. Accordingly, the vibrato oscillator frequency itself is not heard as a note from the speaker 16 and it is only heard as a vibrato of the music or other sound passing through the amplifier. It will be observed that apart from th vibrato oscillator substantially no addition is made to the amplifier itself and the vibrato is therefore accomplished not only with fidelity but with economy. No thumping will be heard in a speaker provided with the above circuit arrangement but the vibrato will be a pure amplitude variation of the otherwise unaffected musical output.

In Fig. 2 is disclosed substantially the same circuit but the speaker field is connected as a filter choke. Thus, the plate and screen currents flowing in the output tubes also flow through the speaker field. Since these currents contain the vibrato frequency, the excitation of speaker field 50 is varied accordingly and the speaker efficiency is caused to vary at the vibrato frequency. Such oscillator signals to the control grids of the output tubes, but the music could be distorted if the tubes were required to operate along a non-linear portion of their operating curves. However, by employing the speaker field as a filter choke, the

speaker actuation by the output tubes is supplemented by the variation of the speaker field currents which do not require abnormal operation of the output tubes. In this manner, the amplitude control may be varied over considerably higher limits. It is also to be understood that the speaker field excitation variation may also be employed independently of the balanced modulator arrangement since such variation may, of itseli.

produce a vibrato through the operation described. In such event, the speaker field is preferably excited by its own, independent power supply, separate from that of the amplifier. The oscillator output is fed to the input grid of a power type tube which, together and in series with the speaker field, is across this power supply. Thus, the tube passes the vibrato modulated currents through the speaker field winding for the abovestated purpose. One or more such power type tubes in parallel may be used.

It will also be noted that in the second embodiment, the oscillator plates and screen currents flow through th speaker field so as to contribute to its efficiency variation at the vibrato oscillator frequency. By the term vibrato oscillator, or vibrato frequency oscillator, I refer to a subaudio oscillator the operating frequency of which is generally below 35 cycles per second.

The balanced modulator arrangement disclosed need not be applied directly to the output stage as will be clear to those skilled in the art, but may be applied to a prior push-pull stage and the amplifier output will likewise be varied in the desired manner.

It is recognized that in the second embodiment the filter condensers 5i and 52 will tend to smooth out the vibrato signals through the speaker field, but such action will not be com plete and the oscillator signals in the output tubes will nevertheless fiow through the speaker field. Filters 51 and 52 may be approximately 8 mid. each and while they are satisfactory for filtering the conventional 60 cycle frequency of the power supply input, they are too small to completely filter the very low oscillator frequency.

While there has been shown what at present are considered preferred embodiments of the invention, it will be evident that many changes and modifications may be made therein without departing from its spirit. It is therefore aimed in the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention.

What is claimed is: 1. In an audio amplifier having a pair of output tubes arranged in push-pull relationship and fed by a pair of oppositely phased audio signals to the respective control grids of the tubes, vibrato means for said amplifier, said vibrato means comprising a vibrato frequency oscillator, a potentiometer across the output of said oscillator so as to vary the output amplitude thereof, said potentiometer being effectively connected to a common midpoint of said control grids so as to feed the output of said oscillator to said control grids in like phase whereby the output of said tubes is amplitude modulated by the oscillator output and whereby the oscillator frequency per se is cancelled in the output of the push-pull circuit, and means to tune said vibrato frequency oscillator over a range of approximately 3 to 20 cycles per second.

2. In an audio amplifier having a pair of audio r output tubes arranged in push-pull relationship for direct connection to an output transformer and fed by oppositely phased audio signals to the respective grids of the tubes whereby said audio signals will be amplified and transmitted to a speaker, vibrato means for said amplifier, said vibrato means comprising an oscillator having an operating frequency of between 3 and 20 cycles per second, said oscillator having the output thereof connected to each of said grids in the same phase whereby said audio signals of opposite phase and oscillator output signals of like phase are simultaneously applied to the respective grids.

3. An audio amplifier including a signal channel terminated by a pair of audio output tubes arranged in push-pull relationship, an output transformer and speaker fed by said tubes, the input of said tubes being fed by respectively oppositely phased audio signals, and vibrato means for said amplifier, said vibrato means comprising an oscillator having an operating frequency of between 3 and 20 cycles per second, and a wire connection from the output of said oscillator to the respective inputs of both of said tubes, said wire connection applying the oscillator output to both of said tube inputs in like phase whereby said tube inputs are both provided with oppositely phased audio signals and similarly phased oscillator output signals.

4. An audio amplifier according to claim 3 and wherein the push-pull relationship of said output tubes comprises respective grids as the input of said tubes, said grids being connected together for simultaneous reception of oppositely phased audio signals, said wire connection being made at a mid-point between said grids so as to apply similarly phased oscillator output signals thereto.

NATHAN I. DANIEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,343,308 Carson June 15, 1920 2,169,762 Kaye Aug. 15, 1939 2,322,884 Roe tkin June 29, 1943 

